The total number of vehicle miles traveled (VMT) on our highways continues to increase across the nation, with people driving greater distances and for longer periods of time. This travel demand consumes large quantities of fuel, corresponding to approximately 50% of our nation's energy costs. Given this steadily increasing travel demand with limited infrastructure growth, highway congestion continues to get worse. There is now a strong need to make our highway travel as efficient as possible. Several studies have been carried out examining roadway congestion in terms of lost productivity and wasted fuel; e.g., in 2003 it has been estimated that 8.7 billion liters of fuel were wasted due to congestion.